Remove drupal_page_footer()

I'll work on this after the kernel patch lands #1463656: Add a Drupal kernel; leverage HttpFoundation and HttpKernel

Yay! It's in :)

The attached patch:

• Eliminates drupal_page_footer() function
• Eliminates call to drupal_page_footer() from drupal_deliver_html_page()
• Modifies drupal_exit() comment block to remove reference to drupal_page_footer()

Thank you, bneil, this looks pretty good.
There's a few more references:

./lib/Drupal/Core/EventSubscriber/RequestCloseSubscriber.php:   *   drupal_page_footer(). There's probably a lot in here that needs to get
./includes/ajax.inc: * This function is the equivalent of drupal_page_footer(), but for Ajax
./includes/ajax.inc: * @see drupal_page_footer()
./includes/bootstrap.inc:  // Called from drupal_page_footer, we write to permanent storage if there

Can the comment in RequestCloseSubscriber be removed?
We probably need another issue to take care of ajax_footer(). The reference to drupal_page_footer() can now probably be removed.
The reference in the comment of bootstrap.inc should also be replaced with something.

Here it is: #1618072: Remove ajax_footer(). That will take care of the two references in ajax.inc. So we shouldn't touch ajax.inc in this issue. Since we're also not touching the TODO, the only missing thing is to rewrite the comment in boostrap.inc.

Something like Called when closing requests, we ...?

This patch incorporates the changes above with the following comment change per Niklas Fiekas:
// Called from drupal_page_footer, we write to permanent storage if there
Changed to:
// Called when closing requests, we write to permanent storage if there

Thanks bneil, that looks good.

Looks good to me.

We likely need another follow-up for drupal_exit().

oops

Committed/pushed to 8.x, thanks!